Electrical relay



y 1942- w. K. SONNEMANN 2,289,149

ELECTRICAL RELAY Filed July 26, 1940 2 Sheets-Sheet 2 Fig. 61

q, 3 F bi3 8 F 0 Amperes o Amperes Restraint Wz ndznys WITNESSES: .9INVENTOR 230 William K.So1memann.

WMAM L 1 3a jib 8b ATT EY Patented July 7, 1942 ELECTRICAL RELAY WilliamK. Sonnemann, Roselle Park, N. J., assignor to Westinghouse Electric &Manufacturing Company, East Pittsburgh, Pa., a corporation ofPennsylvania Application July 26, 1940, Serial No. 347,614

20 Claims.

This invention relates to electrical protective systems, and it hasparticular relation to electrical protective systems employingpercentage differential relays.

In many electrical installations it is desirable to provide a sensitiverelay for removing electrical apparatus from service or for otherwiseprotecting the apparatus when an internal fault occurs therein. If theapparatus is such that electrical current normally enters and leaves theapparatus through a plurality of terminals, the apparatus may beprotected by balancing the currents passing through the terminals.Electrical current passing through the apparatus solely by way of theterminals may be referred to as a through current. For a through currentthe current entering the apparatus equals that leaving the apparatusthrough the terminals and no resultant or unbalance current isavailable. When a fault occurs in the apparatus the current entering theapparatus through the terminals no longer equals that leaving theapparatus through the terminals. Consequently a difference or unbalancecurrent proportional to the current flowing to the fault in theapparatus is obtained by balancing the currents flowing through theterminals, and this difference current may be employed for actuating arelay or other protective device. For example, a relay s actuated may beemployed for removing the app-aratus from service.

In practice, reliance on difference current alone for actuating therelay is inadvisable. This is for the reason that the relay or otherprotective device is energized from the terminals through suitablecoupling devices such as current transformers. Such current transformersgenerally have inherent variations which would provide a secondarydifference current despite the fact that the currents passing throughthe terminals are fully balanced.

A further difliculty resides in the possibility of variations in thedistribution of current in the terminals. For example, current may enterthe apparatus to be protected through a plurality of terminals and leavethe apparatus through only one terminal. Since the current transformerassociated with the last-named terminal must carry a relatively heavycurrent, it follows that this transformer may saturate or operate withdecreasing permeability. Under these circumstances the secondary currentsupplied by this transformer to the relay is unable to balance thesecondary currents supplied by the remaining transformers to the relayand a false operation of the relay or protective device may result. Inother words, the condition of the apparatus or system to be protected isrepresented by the relationship of the currents flowing through theterminals of the apparatus. These currents may be designated primarycurrents. However, the relay determines this relationship by means ofsecondary currents delivered to the relay by the current transformersassociated with the terminals. Consequently, if the secondary currentsfail to represent the primary currents faithfully the relaysinterpretation of the condition of the apparatus may be incorrect, andan unnecessary relay operation may result. The failure of a secondarycurrent to represent a primary current correctly may be for variousreasons such as the saturation of a current transformer caused byexcessive primary symmetrical alternating current or by the directcurrent transient present in certain cases.

In order to prevent false operation of differential relays, it iscustomary to provide such relays with restraint windings. Theserestraint windings are energized in accordance with currents passingthrough the terminals associated with the apparatus to be protected.Consequently, the unbalance or difference current employed forenergizing the relay must be sufficient to overcome the restraintimposed by the restraint windings before the relay can operate. Sincethe restraint increases with an increase in current passing through theterminals, the difference current necessary to operate the relay alsomust increase. The relationship between operating and restraint currentsmay be expressed as a percentage. Therefore such a relay generally isdesignated as a percentage differential relay and sometimes as a ratiodifferential relay.

In a percentage differential relay, the sensitivity of the relay may bedesignated by the per cent of restraint or through current which must bepassed through the operating windings of the relay in order to overcomethe restraint imposed by the restraint windings and actuate the relay.This per cent value varies inversely with the sensitivity of the relay.For example, a five per cent relay is more sensitive than a ten per centrelay when the aforesaid system of rating is employed.

Since the restraint provided by the restraint windings and unbalancecurrents caused by current transformer variations both tend to increasewith an increase in through current, it follows that the restraintwindings tend to prevent false operation of the relay,

When the number of terminals of the apparatus to be protected is large,it is difiicult to provide a single relay structure capable ofeffectively protecting the apparatus. This is for the reason that thenumber of restraint units associated with a single armature assemblyincreases with an increase in the number of terminals and requires anundesirabl heavy armature structure. This difficulty may be overcome byemploying a plurality of differential relay units having their operatingcontacts so connected that all of the units must operate in order toremove the apparatus from service.

The increase in the number of terminals of the apparatus to be protectedcreates an additional problem. The restraint offered by restraintwindings is dependent to some extent on the number of terminals inservice and on the distribution of current in those terminals. Since thepossible restraint variation increases with the. number, of terminals,it follows that the problem is, particularly acute. for apparatus.having a large number of terminals.

The variation in restraint is objectionable. for the. reason that thesensitivity of the relay must be decreased suff ciently so that therelay will not, operate for those normal conditions in which restrain isat a minimum. Lack of sensitivity is objectionable, for the reason thatthe relay fails to respond to small internal fault currents for whichoperation is. desirable.

The variations in restraint, in a differential relay may be appreciablydecreased by increasing the number of restraint windings and connectinggroups of restraint windings for energization in accordance with currentpassing through a single terminal of. the apparatus to be protected.This solution is disclosed in my copending application Serial No.236,396, filed October 22 1938, which has issued as Patent 23 16548.Although this construction represents a marked advance over the priorart, I have found it possible to. increase still further the elliciencyof a differential relay designed for apparatus having a large. number ofterminals.

In accordance with this invention, a plurality of percentagediiferential relay units, each having operating contacts, an operatingwinding, and a plurality of restraint units are, associated forprotective purposes. Each of the restraint units is provided with aplurality of restraint widings. A restraint winding of one of the unitsis connected with the restraint winding of a second relay unit forenergization in accordance with the current passing through a singleterminal. By connecting the restraint windings of the different units tothe same terminals, I have found it possible to decrease the variationsin restraint resulting from variations in the number of terminals'inservice and in the distribution of currents in the terminals.

It is therefore an object of the invention to provide a differentialrelay of improved sensitivity.

It is a further object of the invention to provide a percentagedifferential relay having stable restraint characteristics; for allconditions of service.

It is a further object of the invention to provide a differential relayassembly having a plurality of differential relay units, each includinga restraint winding connected for energization in accordance withcurrentflowing in one terminal of. the apparatusto be protected.

It is a further object of the invention to prolay units RI and R2.

vide a percentage differential relay having a flared characteristic witha reduced range of variation in restraint.

It is a still further object of the invention to provide a differentialrelay having a plurality of relay units for protecting apparatus havinga plurality of terminls wherein the relay has a plurality of parallelbranch circuits each including a restraint winding on each of the relayunits and a current transformer associated with a terminal of theapparatus to be protected.

Other objects of the invention will be ap parent from the followingdescription taken in conjunction with the accompanying drawings inwhich:

Figure 1 is a diagrammatic View of a percentage differential relay forprotecting electrical apparatus.

Fig. 2 is a view in perspective of a diiierential relay unit suitablefor the relay of Fig. i.

Fig. 3 is a diagrammatic View in detail of a modified system forenergizing the operating windings of a percentage differential relay.

Fig. 4 is a graphical representation characteristics of operating andrestraint windings suitable for percentage differential relays.

Fig. 5 is a diagrammatic view of, a percentage differential relayembodying the invention, and

Fig. 6 is a graphic representation of the characteritics of an operatingwinding suitable for a percentage differential relay.

Referring to the drawings, Figure 1 shows a percentage differentialrelay associated with apparatus to be protected. This apparatus maycomprise various devices such as any generator, motor or transformerapparatus having a plurality of terminals associated therewith, but forthe purpose of illustration this apparatus is represented by a bu l. Inthe specific embodiment of Fig. 1 the bus l is provided with sixterminals, 2 to I, each connected to the bus through a circuit breaker2a to la. Although the relays described herein are applicable to variouselectrical systems such as single-phase or polyphase systems, theinvention is described for a three-phase system having three phaseconductors a, b and c In order to simplify the description, differentialrelay protection is disclosed for phase c only. E01 energizing such arelay, phase 0 of each of the terminals is provided with a currenttransformer 2b to lb.

The terminals 2 to 1 represent the terminals of various feeder,generator or tie-circuits or combinations of such circuits which areconnected to the bus. Forexample, theterminals 2 and 3 may represent theconnections of generator circuits capable of supplyingpower' to the bus.The terminalsl, E'and 'lmay represent tie-circuits which are capable ofsupplyingpower to or from the bus as conditions warrant. The terminal tmay represent a feeder circuit for supplying power from the bus.

Protection for the bus I is provided by a percentage diiferentialrelay Rcomprising two re- The relay unitR! includes three pairsof restraintwindings 2d" and 2c, 3d and 3e and 40L and 4e. Similarly the relay unitRZ is provided; with three similar pairs of restraint windings 5d and5c, 6d and 6e, and Id and 16. In addition. the units are provided,respectively, with operating contacts 811 and 8b and withoperating-windings lrla and, 13b.

The relay units R! and R2;mayv be identical. A suitable construction forthe: relay unit RI is illustrated in Fig. 2.

Referring to Fig. 2, the relay unit RI includes an armature assemblyhaving a rotatably mounted shaft 9, an electro-conductive armature diskI0, and an electro-conductive armature disk II. The shaft 9 may carry anarm I2 for operating the contacts 811. Rotation of the shaft in onedirection operates to close the contacts Ba, where as rotation of theshaft in an opposite direction tends to separate the contacts 811.

Rotation of the armature assembly is effected by four electromagnets I3to I6, two of the electromagnets being provided for each of the armaturedisks. One of the electromagnets, such as the electromagnet I3, isprovided with a main pole about which the operating winding I30, islocated. The lower end of the main pole is split and is associated withcooperating poles I and I3d. A short-circuited winding I36 has portionssurrounding the main pole and the cooperating pole I30 and I3d. Whenalternating current II is positioned. The operation of the electromagnetI3 is well known in the art.

The electromagnet I3 is designed preferably to rotate the armatureassembly in a direction to close the contacts 8a. These contactsnormally may be biased to their open condition by 1 means of anadjustable spring I211.

The electromagnet I4 resembles the electromagnet I3. However, theelectromagnet I4 represents a restraint unit, and is provided with twowindings 2d and 26 in place of the one winding I3a of the electromagnetI3. The short-circuited winding 2I of the electromagnet I4 correspondsto the short-circuited winding I Be of the electromagnet I3. when eachor both of the windings 2d and 2e are properly energized by alternatingcurrent, a shifting magnetic field is produced which tends to rotate thearmature disk II]. The electromagnet I4 is designed to produce amagnetic field tending to rotate the armature assembly in a directionsuitable for separating the contacts 811. The electromagnets I5 and I6are similar to the electromagnet I4.

In Fig. 1 it will be noted that the contacts 8a and 8b are connected inseries. For this reason closure of both of the contacts 8a and 8b isnecessary to complete an operating circuit for tripping the circuitbreakers 2a to m or for any other desired operation.

By tracing the circuits in Fig. 1, it may be observed that each of thecurrent transformers is connected to two of the restraint windings. Forexample, the current transformer 2b is connected to the restraintwindings 2d and 36. As a further example, the current transformer 3b isconnected to the restraint windings 3d and 4e. Each of the currenttransformers and its associated restraint windings form one branch of aparallel circuit having seven branches. The operating windings I3a andI3b form the remaining branch of this parallel circuit.

It is believed that the operation of the circuit illustrated in Fig. 1is apparent from the foregoing description. Under normal operatingconditions, the current entering the bus I through the terminals equalsthat leaving the bus through the terminals. Since the current enteringthe bus through the terminals normally equals that leaving the busthrough the terminals, it follows that the currents in the secondariesof the cur- It will be understood that rent transformers 2b to Tobalance and substantially no current flows through the operatingwindings I3a and [31). Consequently the con-. tacts 8a and 81) remainopen.

If a fault external to the bus occurs as at the point X on the terminal6, a substantial fiow of current may pass through the bus towards thepoint X. However, the same condition is present; namely, currententering the bus through the terminals equals that leaving the busthrough the terminals. If the characteristics of the currenttransformers do not vary appreciably under these conditions, andassuming that all current transformers have the same transformationrates, substantially no current flows through the operating windings I3aand I317, and the contacts 8a and 81) remain open. Even though someunbalance be present in the outputs of the current transformers due tosaturation of a transformer carrying appreciable current, such as thetransformer 61), the restraint windings tend to prevent closure of thecontacts 8a and 8b.

If a fault occurs on the bus I as at the point Y, the current enteringthe bus through the terminals no longer equals that leaving the busthrough the terminals. The difference between these currents representsthe current flowing to the fault at the point Y. Since this unbalanceappears in the outputs of the current transformers supplying current tothe differential relay, it follows that a substantial current flowsthrough the operating windings I3a and I3b. Under these conditions thetorques exerted by the operating windings l3a and I3?) on their armatureassemblies should be sufiicient to overcome the restraint torque andshould result'in a closure of the contacts 3a and 8b.

The differential relay illustrated in Figs. 1 and 2 is similar to thatdisclosed in my aforesaid copending application. For a more completedescription of this relay, reference may be made to the aforesaidapplication.

Although the relay illustrated in Fig. 1 is entirely operated as thusfar described, still further improvements may be effected by providingthe relay with a flared characteristic. As disclosed in the copendingapplication of W. K. Sonnemann et al., Serial No. 236,397, filed October22, 1938, which has matured into Patent 2,240,677, a flaredcharacteristic may be obtained by energizing the operating winding of apercentage differential relay through a transformer which saturates oroperates with decreasing permeability as the energizing currentincreases. Similar characteristics may be provided for the relay unitsRI and R2 by design ing the electromagnets for the operating windingsI3a and I3b to saturate or operate with decreasing permeability as theirenergizing currents increase.

Connections employing saturating auxiliary transformers for providing aflared characteristic are illustrated in Fig. 3 wherein the unbalance ordifference current is conducted from the restraint windings through theprimary of two auxiliary current transformers 22 and 23, Thesetransformers are designed to saturate or operate with decreasingpermeability when their energizing currents rise above predeterminedvalues. The operating windings I3a and I3b are connected respectivelyacross the secondaries of the transformers 22 and 23. In the specificillustration, the auxiliar transformers are autotransformers havingadjustable taps 22a and 23a. This construction provides a desirablecharacteristic for the reason that after saturation theauto-transformers operate somewhat like imped ance voltage dividers.

The effect of the flared characteristic is to make the relay moresensitive for low values of restraint current (such as valuescorresponding to normal load currents for the system) than for highervalues of restraint current. Consequently the relay trips properly oninternal faults involving small current magnitudes. At the same time therelay does not trip for external faults involving large currentmagnitudes, which cause one or more of the terminal current transformersto depart substantially from their correct transformation ratios.

It should be noted that by employing only one of the restraint windingson each of the restraint electromagnets, the relay illustrated in Fig. 2may be employed in a conventional manner for protecting an electricalsystem having three terminals.

It will be noted that a variet of conditions may obtain for the busshown in Fig. 1. For example, current may be supplied to the bus throughfive terminals, 2, 3, l, 5 and l, and may leave the bus through a singleterminal 6. Furthermore, certain of the terminals may be out of serviceunder certain conditions. Because of these and other variations, therestraint offered by the restraint windings is not always the same for agiven total current flow. This may be illustrated more clearly by anumerical example.

Let it be assumed that current leaves the bus I through a; singleterminal 5 to a fault at the point X, and that the current through theterminal 6 has a value of '75 amperes referred to the secondary of thecurrent transformer 51?. Let it be assumed further that the currenttransformer 6b saturates so that it delivers only 65 amperes. Since thecurrent supplied to the bus is divided among a number of terminals, itmay be assumed that the remaining current trans-' formers hold theirratios.

The operating characteristics of the operating and restraining windingsare illustrated in Fig. 4 wherein ordinates represent torque incentimeter grams developed by the windings, and abscissae representamperes' at 60 cycles supplied to the windings. Curves A and B representan operating winding and a restraint winding respectively. Bydetermining the current supplied to each winding for various connectionsof the terminals supplying current to the fault on the terminal 6, forthe specific example, it will be found that the resta'ining torque inthe relay unit developing the maximum restraining torque will vary fromapproximately 43 to 105 centimeter grams. This means that thedifferential relay must be made sufficiently insensitive so that theoperating winding of each relay unit will not develop a torque of morethan 48 centimeter grams under external fault conditions due to theunbalance currents resulting from saturation of current transformers orfrom other reasons.

If the'same calculations are carried out for an internal fault as at thepoint Y, it will be found that the restraining torque will vary fromapproximately 50 to 105 centimeter grams depending upon the distributionof current in the terminals. ihese computations are again made forafault current of 75 amperes referred to the secondary of a currenttransformer.

In order to be sure of positive tripping for the internal fault, itfollows that the differential relay must have a sensitivity sufficientto develop over 105 centimeter grams of operating torque. Thissensitivity is required despite the fact that under certain currentdistributions about 51 centimeter grams of operating torque suffice to'actuate the difierential relay.

Although the differential relay illustrated in Fig. 1 is verysatisfactory, I have found it possible to decrease still further thevariations in restraining torque resulting from the variations incurrent distribution. A differential relay arranged in accordance withthe invention is illustrated in Fig. 5.

Referring to Fig. 5, the two relay units RI and R2 are connected to thecurrent transformers 2b to lb in a somewhat different manner. In Fig. 1the restraint units of each relay unit are associated with only threeterminals. In Fig. 5, on the other hand, the restraint units of eachrelay unit are associated with all of the current transformers. Thismeans that each current transformer is connected to a restraint windingin each of the relay units.

By inspection of Fig. 5, it will be observed that the restraint windingson each restraint unit are connected to different current transformers.The pairing of current transformers preferably diifers for eachrestraint unit. Moreover, each of the current transformers is associatedwith a different group of restraint units. In Fig. 5, a seven branchparallel circuit is illustrated in which each of six branches includes acurrent trans former and a restraint winding positioned in each of therelay units. For example, the current transformer 21; is connected inseries with the restraint winding 2d and the restraint winding 1d. As afurther example, the current transformer 3b is connected in a seriescircuit including the restraint winding 26 and the restraint winding 5c.The operating windings 13a, l3?) are in the seventh branch of theparallel circuit. Although the relay of Fig. 5 may be employed without aflared characteristic, preferably such a characteristic is provided.

The improvement effected by redistributing the restraint windingsrelative to the current transformers may be shown by making equivalentcomputations for the relay of Fig. 5. These computations again are basedon a total fault current of '75 amperes referred to the secondary sideof the current transformers. For the external fault at the point X, therelay of Fig. 5 is found to have a restraint in the relay unitdeveloping maximum restraint torque which Varies from 42 to centimetergrams. It will be observed that for external faults the relay of Fig. 5is slightly better than the relay of Fig. 1 through having a smallerrange between maximum and minimum restraint torques.

For an internal fault at the point Y, of '75 amperes referred to thesecondary side of the current transformers, it is found that therestraint torque varies only from 44 to 50 centimeter grams. This meansthat to assure tripping on suchan internal fault the operating windingsof the relay units need develop a torque of but slightly over 50centimeter grams. Because of thisgreat reduction in required torque, thedifferential relay of Fig. 5 may be made much more insensitive than therelay of Fig. 1 to external faults, without impairing the protection forinternal faults. Greater insensitivity to external faults is desirablefor the reason that the relay should not operate for such faults. Thegreater insensitivity is an additional safeguard against false relayoperations caused by faulty current transformer performance.

As illustrated in Fig. 5, the contacts 8a and 82) may be employed forconnecting a source of power (represented by polarity markings) to asuitable translating device 24 which may be employed for actuating tripcoils for the circuit breakers, alarm devices or any other suitableprotective devices.

In some special cases discrimination should be exercised in connectingthe relay of Fig. to the system to be protected. For example, if theterminals 2, 4 and 6 represent tie lines connected to a distantsubstation bus, a fault on the substation bus might producesubstantially equal currents flowing through the terminals 2, 4 and 6.If current supplied to the bus I through the terminals 3, 5 and 1divides substantially uniformly among the terminals 3, 5 and 1, it willbe noted that substantially no restraint will be offered in either ofthe relay units RI or R2. For example, since current flows in oppositedirections through the terminals 2 and 3, the current passing throughthe winding 2d neutralizes the effect of the current passing through therestraint winding 26. A similar condition will be found to obtain foreach of the remaining pairs of the restraint windings. For this reasoncare should be exercised to prevent the cancellation of all restraint.In the particular example cited, the cancellation of restraint may beprevented by connecting the terminals 5, E and I to the distantsubstation bus.

As an example of a relay application wherein a tates proper operation ofthe relay under such 7 exacting conditions.

By adjusting the bias exerted by the spring 12a, the minimum sensitivityof the relay units may be varied to meet varied local conditions. Forexample, if a power transformer is located in the zone to be protected amoderate spring bias may be employed for decreasing the tendency of therelay to trip on magnetizing inrush current required by the transformer.

Typical characteristics of an operating winding [3a, or l3b areillustrated in Fig. 6 wherein abscissae represent current flowingthrough the energizing circuit for the operating winding and ordinatesrepresent torque available for overcoming the torque of the restraintwindings. Two curves 25 and 26 are shown. The curve 25 represents a tapconnection for a saturating auxiliary transformer 22 or 23 such that 12amperes through the energizing circuit for the operating winding justsuffices to overcome a restraint torque of 70 centimeter grams. In thespecific relay represented by the curves, such a restraint torque isobtained from a heavy internal fault. Similarly, the curve 26 representsa tap connection such that 38 amperes through the energizing circuit forthe operating winding suffices to overcome the restraint torque of '70centimeter grams. Other taps may be provided as desired.

Fig. 6 is purposely drawn to show low current values. It will beobserved that for very low current values the curves exhibit a verysmall spread. In other words, all taps provide substantially the samesensitivity for small internal faults. For larger fault currents thecurves show an appreciable spread. The selection of the proper tap foroperation is determined by local conditions.

The displacement of the curves from the intersection of the axesrepresents the effect of the torque exerted by the spring I 2a. In somecases this spring torque advantageously may be increased. For example,if a power transformer is within the system to be protected, substantialmagnetizing inrush current may be required by the transformer. Anincreased spring torque, especially when provided in a relay having aflared characteristic, will reduce the tendency of the differentialrelay to trip in response to the magnetizing inrush current.

Although the invention has been described with reference to certainspecific embodiments thereof, it is obvious that numerous modificationsare possible. Therefore, the invention is to be restricted only by theappended claims when interpreted in view of the prior art.

I claim as my invention:

1. In a relay system, a plurality of restraint units, a plurality ofenergizing sources for said restraint units, each of said energizingsources being connected to energize a different pair of said restraintunits, a separate operating unit for each restraint unit of one of saidpairs of restraint units, and control means responsive to the combinedconditions of each of said operating units and its associated restraintunit.

2. In a differential relay system, three restraint units, two windingsfor each of said restraint units, six energizing sources, each of saidsources being connected for energizing a separate one of said windings,and a common operating unit for said restraint units.

3. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a plurality of groups of restraint means, each of saidgroups of restraint means being connected for energization in accordancewith the current flowing through one of said terminals, separateoperating means for each restraint means in one of said groups connectedfor energization in accordance with current leaving said system by apath other than said terminals, and control means responsive to thecombined conditions of said restraint and operating means.

4. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a plurality of groups of restraint means, each of saidgroups of restraint means being connected for energization in accordancewith the current flowing through a separate one of said terminals,separate operating means for each restraint means in one of said groups,said operating means being connected for energization in accordance withcurrent leaving said system by a path other than said terminals, andcontrol means responsive to the combined conditions of said restraintand operating means, said means being so proportioned that the ratio ofenergization of said operating means to the energization of saidrestraint means necessary to effect an operation of said control meansincreases with an increase in energization of said restraint means.

accordance with current leaving said system by a path other than saidterminals, and control means responsive to the combined conditions ofsaid restraint and operating means.

6. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a plurality of groups of restraint means, each of saidgroups of restraint means being connected for energization in accordancewith the current flowing through one of said terminals and each of saidrestraint means being connected for energization in accordance withcurrents flowing in a, distinct group of said terminals, separateoperating means for each restraint means in one of said groups, saidoperating means being connected for energization in accordance withcurrent leaving said system by a path other than said terminals, andcontrol means responsive to the combined conditions of said restraintand operating means, said means being so proportioned that the ratio ofenergization of said operating means to the energization of restraintmeans necessary to effect an operation of said control means increaseswith an increase in energization of said restraint means.

7. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of groups of restraintmeans, each of said groups of restraint means being connected forenergization in accordance with the secondary current flowing in one ofsaid current transformers, separate operating means for each restraintmeans in one of said groups, said operating means being connected forenergization in accordance with current flowing to a fault occurring insaid system, and control means responsive to the combined conditions ofsaid restraint and operating means.

8. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of groups of restraintmeans, each of said groups of restraint means being connected forenergization in accordance with the secondary current flowing in aseparate one of said current transformers, separate operating means foreach restraint means in one of said groups, said operating means beingconnected for energization in accordance with current flowing to a faultoccurring in said system, and control means responsive to the combinedconditions of said restraint and operating means, said means being soproportioned that the ratio of energization of said operating means tothe energization of restraint means necessary to effect an operation ofsaid control means increases with an increase in energization of saidrestraint means.

9. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a, source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of groups of restraintmeans, each of said groups of restraint means being connected forenergization in accordance with the secondary current flowing in one ofsaid current transformers, and each of said restraint means beingconnected for energization in accordance with currents flowing in thesecondary windings of a distinct group of said current transformers,separate operating means for each restraint means in one of said groups,said operating means being connected for energization in accordance withcurrent flowing to a fault occurring in said system, and control meansresponsive to the combined conditions of said restraint and operatingmeans.

10. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of groups of restraintmeans, each of said groups of restraint means being connected forenergization in accordance with the secondary current flowing in one ofsaid current transformers, and each of said restraint means beingconnected for energization in accordance with currents flowing in thesecondary windings of a distinct group of said current transformers,separate operating means for each restraint means in one of said groups,said operating means being connected for energization in accordance withcurrent flowing to a fault occurring in said system, and control meansresponsive to the combined conditions of said restraint and operatingmeans, said means being so proportioned that the ratio of energizationof said operating means to the energization of said restraint meansnecessary to effect an operation of said control means increases with anincrease in energization of said restraint means.

11. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of differential relayunits each comprising control means, restraint units each having aplurality of restraint windings and an operating winding for actuatingsaid control means, means for connecting restraint windings on differentrelay units for energization from the secondary of one of said currenttransformers, means for energizing each of said operating windings inaccordance with current flowing to a fault occurring in said system, andcontrolled means operable to a predetermined condition by said controlmeans only when all of said control means are in their controlconditions.

12. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of differential relayunits each comprising contact means, a plurality of restraint windingsand an operating winding for controlling said contact means, means forconnecting a separate group of restraint windings for energization fromeach of said current transformers, the restraint windings of each ofsaid groups being associated with different relay units, means forenergizing each of said operating windings in accordance with currentflowing to a fault occurring in said system, said contact means beingconnected in series for effecting a control operation, and each of saidrelay units being so constructed that the ratio of energization of itsoperating winding to the energization of its restraint windingsnecessary to effect an operation of said contact means increases with anincrease in energization of said restraint windings.

13. In a protective arrangement for an alternating current electricalsystem having a plurality of terminals through which currents normallyenter and leave said system, a current transformer for each of saidterminals for providing a source of current dependent on the currentflowing through the terminal associated therewith, and a plurality ofdifferential relay units each having a plurality of pairs of restraintwindings, said restraint windings being arranged in groups eachcomprising arestraint winding on each of said relay units, having anoperating winding connected for energization in accordance with currentflowing to afault occurring in said system, and having contact meanscontrolled by said restraint and operating windings, the contact meansof said relay units being connected in series, and means for connectingeach of said groups for energization from a separate one of said currenttransformers, said relay units being so proportioned that the ratio ofenergization for said operating windings to the energization for saidrestraint windings necessary for actuating said contact means increaseswith increase in the energization of said restraint windings.

14. In a protective arrangement for an electrical system having aplurality of terminals 6 through which currents normally enter and leavesaid system; a plurality of differential relay units each comprising anelectroconductive armature assembly mounted for rotation,electromagnetic operating means inductively associated with saidarmature assembly, said operating means being effective when energizedfor urging said armature assembly in a first direction of rotation, anda plurality of restraint means each including an electromagnetinductively associated with said armature assembly and having aplurality of energizing windings, said electromagnet being effectivewhen energized for opposing rotation of said armature assembly in saidfirst direction;

means for connecting a group of said windings tromagnet 15. In aprotective arrangement for an electrical system having a plurality ofterminals through which currents ormally enter and leave said system andhaving a current transformer associated with each of said terminals forproviding a source of current dependent on the current flowing throughthe associated terminal; a plurality of differential relay units eachcomprising an electroconductive armature assembly mounted for rotation,electromagnetic operating means inductively associated with saidarmature assembly, said operating means being effective when energize-dfor urging said armature assembly in a first direction of rotation, anda plurality of restraint means each including an electromagnetinductively associated with said armature assembly and having aplurality of energizing windings, said electromagnet being effectivewhen energized for opposing rotation of said armature assembly in saidfirst direction; means for connecting a group of said windingscomprising one of said windings on each of said relay units forenergization in accordance with current passing through a separate oneof said current transformers; means connecting said operating means forenergization from said current transformers in accordance with afunction of the difference between currents entering and leaving saidsystem through said terminals, and means effective for initiating acontrol operation only when each of said armature assemblies is in apredetermined position.

16. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system and having a current transformer associated with each ofsaid terminals for providing a source of current dependent on thecurrent flowing through the associated terminal; a plurality ofdifferential relay units each comprising an electroconductive armatureassembly mounted for rotation, electromagnetic operating meansinductively associated with said armature assembly, said operating meansbeing effective when energized for urging said armature assembly in afirst direction of rotation, and a plurality of restraint means eachincluding an elecinductively associated with said armature assembly andhaving a plurality of energizing windings, said electromagnet beingeffective when energized for opposing rotation of said armature assemblyin said first direction; means for connecting a group of said windingscomprising one of said windings on each of said relay units forenergization in accordance with current passing through a separate oneof said current transformers; means connecting said operating means forenergization from said current transformers in accordance with afunction of the difference between currents entering and leaving saidsystem through said terminals; and means effective for initiating acontrol operation only when each of said armature assemblies is in apredetermined position; each of said relay units being so proportionedthat the ratio of energization of its operating means to energization ofits restraint means necessary to actuate its armature assembly to saidpredetermined position increases with an increase in energization ofsaid restraint means.

17. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system and having a current transformer associated with each ofsaid terminals for providing :a "source of current dependent on the cur-.rentiflowing through the iassociatedterminal; a

plurality of differential relay'units each comprising .an.electroconductive armature assembly m-ountedfor rotation,electromagnetic operating meansinductively associated with said armatureassembly, :saidoperating means being effective when energized for urgingsaid armature assembly in a firstdirectiono'f rotation, and a pluralityof restraintmeans each including anelec- .tromagnet inductivelyassociated with said armatureassembly and 'having'a-plurality ofenergiz- .ing windings, said electromagnet being effective whenenergized for opposing rotation of said armature assembly in said firstdirection; means for connecting a group of-said windings comprising -oneof said win-dings on each of said relay units for energization inaccordance with current passing through a separate one of said currenttransformers, the grouping of current transformers employedforenergizing each one of said electromagnets differing from thegrouping of current transformers employed forenergizing each of theremaining electromagnets, means-connecting said operating means forenergization from said current transformers in accordance with afunction of the difference between currents entering and leaving saidsystem through said terminals, and means effective for initiating acontrol operation only when each of.

said armature assemblies is in a predetermined position.

18. .In a protectivearrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, a current transformer for each of said terminals forproviding a source of current dependent on the current flowing throughthe terminal associated therewith, a plurality of difsaid system, andcontrolled means operable to a predetermined condition by said controlmeans only-when all of said control means are in their controlconditions.

19. In a protective arrangement for an elec- .trical system having aplurality of terminals through which currents normally enter and leavesaidsystem, ;a current transformer for each of said terminals forproviding a source of current dependent ,on the current flowing throughthe terminal associated therewith, a plurality of differential 'relayunits each comprising control means, restraint units each having aplurality of restraint windings and an operating winding for actuatingsaid control means, means for establishing an electrical circuit havinga plurality of parallel branches, each of said branches including arestraint'winding on each of said relay units and a secondary winding ofone ,of said current transformers, means for connecting said operatingwindings .in parallel with said branches, and controlled means operableto a predetermined condition by said control means only when all of saidcontrol means are in their control conditions.

20. In a protective arrangement for an electrical system having aplurality of terminals through which currents normally enter and leavesaid system, current-transformers associated with said terminals, adifferential relay having operating means effective when energized forurging said relay to a first condition, said relay having a plurality ofrestraint means effective when energized for opposing operation of saidrelay to said first condition, means connecting each of said restraintmeans for energization in acc0rdance with current passing through aseparate one of said terminals, and means for energizing said operatingmeans, said last-named energizing means comprising a currentauto-transformer designed to saturate within the expected range ofenergization thereof, means connecting said operating means forenergization in accordance Withthe output of said auto-transformer, andmeans including said current transformers associated with said terminalsfor energizing said auto-transformer substantially in accordance withthedifierence between currents entering and leavingsaid system throughsaid terminals.

WILLIAM K. SONNEMANN.

